Allow using the search path from -resource-dir with emcc -print-file-name

[Diyou]

This allows emcc to change the search path when running it like: emcc -resource-dir=... -print-file-name=... which mimics the upstream clang behavior.

[sbc100]

Can you share the exact command line that cmake is using to ask for this path? I don't think this change should be necessary but perhaps I don't understand.

[sbc100]

I think all we need to do here is make sure that libc++.modules.json is installed in the search dirs that emcc already uses.

$ emcc -print-search-dirs
programs: =/usr/local/google/home/sbc/dev/wasm/llvm-build/bin
libraries: =/usr/local/google/home/sbc/dev/wasm/emscripten/cache/sysroot/lib/wasm32-emscripten

If you install libc++.modules.json in ..emscripten/cache/sysroot/lib/wasm32-emscripten then it can be found by emcc -print-file-name.

$ emcc -print-file-name=foo.txt
foo.txt

$ touch /usr/local/google/home/sbc/dev/wasm/emscripten/cache/sysroot/lib/wasm32-emscripten/foo.txt

$ emcc -print-file-name=foo.txt
/usr/local/google/home/sbc/dev/wasm/emscripten/cache/sysroot/lib/wasm32-emscripten/foo.txt

[Diyou]

Can you share the exact command line that cmake is using to ask for this path?

It usually just runs em++ -print-file-name=libc++.modules.json which at the moment just prints the filename without the expected path.

execute_process(
    COMMAND
      "${CMAKE_CXX_COMPILER}"
      ${CMAKE_CXX_COMPILER_ID_ARG1}
      -print-file-name=libc++.modules.json
    ...

CMAKE_CXX_COMPILER_ID_ARG1 can be used here to add more flexibility.
Upstream clang allows something like
clang++ -resource-dir=<...> -print-file-name=libc++.modules.json

But yeah we probably should build and install the files properly anyways. Not too familiar with how system_libs.py works but I'm taking a look.

[sbc100]

Can you move this lines inside the if print_file_name block?

Then you can do something like this:

resource_dir_args = [a for a in args if a.startswith(('-resource-dir=', '--resource-dir='))]
if resource_dir_args:
    resource_dir = resource_dir_args[-1].split('=')[1]
else:
    resource_dir = cache.get_lib_dir(absolute=True)
fullpath = os.path.join(resource_dir, libname)

[sbc100]

Can you change this to emcc.py. The file emcc only exists on UNIX. On windows its called emcc.exe. But emcc.py always exists.

[sbc100]

Can you update the PR title and description. This is really about adding support for -resource-dir I think?

Its not really specific to cmake and IIRC we don't want to be passing a custom -resource-dir in #27065 do we?

[Diyou]

IIRC we don't want to be passing a custom -resource-dir in #27065 do we?

Yeah but this can be used when testing until we have code to install the files propery